Pulse time jitter measuring device



March 1o,I 1959 Filed May 2l, 1954 LST Emu/l.

W. T. POPE, JR

PULSE TIME JITTER MEASURING DEVICE 54 Sheets-Sheet 1 v JNVENToR. WILLIAMT POPEWiR Blfdmw l AGENT-4 A-ruEvE-r? f Filed May l21, 1954 w. T. POPE,JR

PULSE TIME JITTR MEASURING DEVICE 3 Sheets-Sheet 2 WILLIAM INVENTOR. IPopE, Je.

AGENT- 3 Sheeis-Sheet S IN V EN TOR WILLIAM 1f POPE, JR. )la

w. T. POPE, JR

PULSE TIME JITTEE MEASURING DEVICE March 10, 1959 Filed May 2l, 1954United 2,877,414 PULSE TIME JI'ITER MEASURING DEVICE William T. Pope,Jr., Rome, N. Y., assigner to the United States of America asrepresented by the Secretary of the Air Force The invention describedherein may be manufactured and used by or for the Government forgovernmental purposes without payment to me of any royalty thereon.

Present day applications of pulse techniques often place stringentrequirements on the accuracy of the time interval between pulses. Anexample is MTI (moving target indication) radar. In such applicationsspurious variations in this time interval, referred to in the art astime jitter, aiect the accuracy of the results obtained and it istherefore desirable that they be reduced to a minimum.

It is the object of this invention to provide means for measuring smallamounts of time jitter in the repetition interval of a train of pulses.Time jitter may be introduced by the pulse train generating apparatus,and also fby any apparatus through which the pulse train passes. It isaccordingly a further object of the invention to provide means formeasuring not only the overall time jitter but also the incrementaljitter added to a pulse train by its passage through an amplifier,modulator or other circuit4 stage. yOverall jitter is measured,inaccordance with the invention, by comparing the phase of a continuouswave generated by a stable reference oscillator with that of anoscillator of the same frequency keyed by the pulse signal. In measuringincremental jitter the phases of two keyed oscillators arecompared, oneoscillator being keyed by the pulses entering and the other by thepulses leaving the stage being checked. The sensitivity ofthe Patentmeasuring device depends upon the frequency of the two oscillators.

A more detailed description of the measuring device will be given inconnection with the specific embodiments thereof shown in theaccompanyingk drawings,y in which:

Fig. l shows waveforms occurring in the4 measurement of overall timejitter;

Fig. 2 shows the idealized characteristic of a phase detector;

Fig.v 3 illustrates the pattern obtainedon the screen of an oscilloscopein making jitter measurements;

.Fig 4 shows the actual characteristic of a phase detester; 4 Fig. Sis ablock diagram of an overall time jitter measuring device in accordancewith the invention;

Fig. 6 shows the circuit details of various elements of Fig. 5;

Fig. 7 is a block diagram of an incremental time jitter measuring devicein accordance with the invention; and

Fig. 8 shows waveforms appearing in the apparatus of Fig. 7.

Referring to the drawing, Fig. 1 illustrates the procedure for measuringoverall time jitter in the train of pulses shown at (a) and having arepetition period T. The pulses initiate square waves which key aselffexcited oscillator having an angular velocity wo and a xed startingv phase. relative to the square wave` The keyed output of thisoscillator is shown at (b) and has the form 2,877,414 Paten-ted Mar. 1o,y195s where A0 isl the initial phase angle. The output of a continuouswave reference oscillator having an angular velocity of wc is shown at(c) and has the form where Ac is the initial phase angle.

In the absence of time jitter any pulse Tn of the train will occur atthe timet=NT, N being any integer. In the presence of jitter this pulsewill occur at a time that differs from NT'by the amount of time jitterAT. Fur ther, in the absence of jitter and with fc and fo equal and amultiple of the phase difference between wn and wc will be constantthroughout all the intervals T', while in the presence of jitter thisphase dilierence will difer from the above constant value by an amountproportional to the jitter AT. This may be shown as follows: Consideringany pulse Tn of the pulse train, the phase dilference A between w and wcduring the interval Tv corresponding to Therefore, it is seen from (5)that if AT=0, A is constant and equal to l0-Ac; and that if AT%0, Adiffers from the con'stant value Ao-Ac by an amount proportional to AT.Since time jitter is a rapidly varying phenomenon, AT will be a varyingquantity causing a corresponding uctuation of A fromv interval tointerval about the constant phase difference A0-Ac- The value ofthisdeviation at the beginning of any interval T kis given by the expressionf drived from Equation 5.

Fig. 2 shows the idealizedcharacteristic of a phase de tector. If twowaves wo and wc of the same frequency are applied to such a detector theoutput will correspond to the phase difference A between the'two waves.The output varies from a maximum at A=0 to a minimum at A=1r. If theinitial phases A0 and Ac, Equation 5, are so selected that Ao-A Theoperating point mayalso be established at plitude Em, may be usedas 'ameasure of the time jitter AT.Y This maybe shown as follows: From Fig.2, 5

AE AA 7) E-T substituting the value of AA from Equation 6,

AE 21.1( 10 (8) EW--T-AT-zfnr from which AE 1 (9) AT--E'R l5 or AE' 1 9AT'==--- a) Eaafc where AE' is the range of uctuation of AE and T' isthe corresponding range of fluctuation of AT.

In making actual measurements of time jitter-the output of the phasedetector is applied to an oscilloscope which gives a pattern similar tothat shown in Fig 3. By slightly detuning the reference oscillator thephase diiereuce A between w0 and wc may be made to vary slowly between Oand 1r and the limits E max and E min es- 1. tablished, from which Eppmay be determined. E max and E min may also be determined by adjustingthe phase dilerence betweenwo and wc, using a suitable phase shifter, tovalues of 0 and and denoting the difference in detector outputs at thetwo phase differences. With the frequency fc of the reference oscillatorat itsiproper value the fixed phase dilerence .4o-Ac between we wc isadjusted to 1r 311' (or -2- radlans so that the amplitude of thedetector `output is approximately midway between E max and E min. AE,the

range of fluctuation of E, is then determined by measuring the range ofdetector output amplitude fluctuation. o Knowing Epp, AE' and fc, AT',the range of uctuation of AT, may be computed from Equation 9a, whichhowever assumes the ideal detector characteristic` of Fig. 2. Since thischaracteristic is not attained in practice, but one more like that shownin Fig. 4, AT should be multiplied by a correcting factor F, less thanunity, derived from the actual detector characteristic by extending itslinear portions and equal to i as shown in Fig. 4. For highest accuracyshould be confined to jitter amplitudes AE' thirds the amplitude of thetime base Epp.

Fig. 5 is the block diagram of a jitter measuring device operating onthe above principle. The pulse U'ain to be checked is applied toinput'terminal 1 and thence through amplifier and shaper 2 and cathodefollower 3 to onecycle multivibrator 4 which is, triggered by the pulsesand produces square waves o f" approximately 50 microsec- 7o ondsduration., having leading edges coincidentwith the leading edges of thepulses. -The squarewaves are applied through cathode follower 5 tooscillator 6 and key 1. 9 this oscillator, operating atfrequency fo, toproduce the train of high frequency pulses shown in Fig. 1(b) These 75measurements less than twooscillators 6 and 9 must pulses are appliedthrough amplifier 7 to input #2 of phase detector 8.

The continuous wave reference frequency fc is generated by oscillator 9and applied through adjustable phase shifter 10 and amplifier 11 toinput #1 of the phase detector. The output of the phase detector, whichis proportional in amplitude to the phase diierence` between the wavesapplied to its two input terminals, is applied through cathode follower12 to cathode ray oscilloscope 13, the sweep of which is synchronizedwith `the original pulse input. Phase shifter 10 is utilized to set thevalue A of the tilted phase diierence between the two waves (Afr-Ac inEquation 5) and to thereby select the desired operating point on the-detector characteristic.

Suitabledesigns for elements 29, 11 and 12 of. Fig. 5 are shown in Fig.6. The pulse amplifier and shaper 2 comprises phase splitter stage 14,amplier 15 and clipper 16a. `Phase splitter stage 14 has switch `S1associated therewith so that input pulses'of either Vpolarity maybeemployed. The cathode follower 3 is tube 16b in Fig. 6. One-cyclemultivibrator 4, cathode follower 5, keyed oscillator 6 and bufferamplier 7 are the stages comprising tubes 17, 18, 19 and 20,respectively, of Fig. 6.

Reference oscillator 9 `may be a crystal oscillator such as shown inconnection with tube 21 while stage 22 corresponds to the buieramplifier 11 of Fig. 5. The phase detector 8 of Fig. 5 may be ofthegated beam type shown in Fig. 6 as tube 23 and associated circuit, amore detailed description of which may be found in General ElectricEngineering Bulletin ET-BZS. Cathode follower stage 12 of Fig. 5 may bea stage such as that in Fig. 6 containing tube 24. Phase shifter 104 maybe of any suitable design capable of shifting phase up to slightlymorethan 90.

Thechoice of the reference oscillator frequency depends on the order ofmagnitude of the jitter which is' to be measured. The smaller the amountof jitter the higher the frequency of the reference oscillator shouldbe. Amplitude jitter can be observed on the oscilloscope down to 1,60 ofthe Em, amplitude. Referring to Equation 9, if f0=10 mc. jitterdown to 11 @XX- .001 microseconds may be discerned. The maximum jitter that maybe measured with this reference frequency is -21=.05 microseconds 5Lowering the reference frequency to 5 mc. permits measurement of jitterfrom 0.002 to 0.1 microsecond.

The time-on of the keyed oscillator should be suflicient f, to allow theoscillator to stabilize after the initial switching transient and alsoto give a video output pulse of convenientwidth for viewing on theoscilloscope. A For reference frequencies ofthe above order and pulserepetition frequencies of from 2 to 4 kc., a time-on of50 5,microseconds is satisfactory. In order to minimize lthe starting timejitter, the multivibrator triggering lpulse should have a very fastrise' time from a low impedance source.` In order to permit adjustmentof fo and fo to an integral multiple of the pulse repetition frequency,l be tunable over a'smallf'range as provided for by condensers 25 and26, respectively. The frequency range over which the oscillators must betunable to meet this condition is f j PRF i 2 In measuring incrementaljitter an arrangementrsuch as shown in Fig.- 7 is used. The inputpulsesapplie'd to stage,` 30, the stage under test, are also applied'through variable'delay A431 to keyedoscillator channel #-1. 'Simitlarly the output pulses of stage `are applied through variable delay 32`toky'ied oscill'atorchannel #2. The

two keyed oscillator channels are identical and each has the samecircuitry as that shown preceding detector input #2 in Figs. 5 and 6.The output of keyed oscillator channel #1 is applied to input #1 ofphase detector 33 and the output of keyed oscillator #2 is applied tothe #2 input of the detector. The two outputs are of the same form, eachconsisting of a series of A. C. pulses having their leading edgescoincident with the leading edges of the input or output pulses of thestage 30. The duration of the A. C. pulses is such that the A. C. pulsesgenerated by corresponding input and output pulses of the stage undertest overlap inu time. The output of the phase detector 33 is applied tothefos'c'illosop'efft from which the time jitter may be determined asexplained above in connection with the overall jitter measuring device.`v

The above is illustrated more clearly in Fig. 8. A pulse arriving at theinput of stage 30 at To, shown at (a), will arrive at the output of thestage after anv interval equal to tri-Atl, where t1 is the fixed delaythrough the stage and Atl is the time jitter introduced by the stage.The pulse at (a) generates the square wave at (b) which keys theoscillator in channel #1 to produce the wave at (c). Similarly theoutput pulse atA (d) acts in channel #2 to generate the square wave at(e) and the keyed oscillator output wave at (f). As shown in Fig. 8, theduration Ts of the square waves is made greater than tl-l-Atl so thatthe waves at (c) and (f) overlap in time. This overlap should besuieient for proper operation of the phase detector and such as toprovide a video pulse of suitable width for viewing on the oscilloscope.

If the frequency setting of the keyed oscillators is such that w1=w2=w0,then the phase difference A between the waves w1 and W2 in Fig. 8(c) and(f) is where z' is measured from the leading edge of the pulse in Fig.8(d) and A1 and A2 are the initial phases of W1 and W2.

Equation 10 may be simplified to (11) =1Az+wo1+wo^f1 from which, sinceA1, A2 and wotl are constants,

:WOAI

As seen from Equation 1l, when the time jitter At1=0, A is a constantequal to Al-Ag-l-wotl. A1 and A2 are controlled by delays 31 and 32,respectively, in Fig. 7, and wutl is determined by the frequency fo. Byproper selection of these parameters Al-A-l-wutl may be made equal to soas to establish the operating point at the center of the linear portionof the phase detector characteristic as shown in Fig. 2. In the presenceof jitter the amplitude of the detector output fluctuates about thedetector output at At1=0. The range of this fluctuation AE may bemeasured on an oscilloscope and compared with the measured value of Epp,Fig. 2, to arrive at the corresponding overall time jitter Atl. Therelationship is -i- (13) At, EW 2f0F derived in the same manner asEquation 9a. F is the al ready discussed correction factor derived fromthe actual detector characteristic as explained in connection with Fig.4. The value of Epp may be determined by slightly detuning one of thekeyed oscillators or by means of variable delays 31 and 32.

The frequency fo is determined by the minimum and maximum jitter to bemeasured as already explained. Smaller amounts of jitter require higherfrequencies which in turn reduce the maximum amount of jitter that maybetween channels .t o 2 r 2 The above described jitter checker isparticularly useful in measuring the incremental time jitter produced byhydrogen thyratron modulators used in MTI radar equipment. Hydrogenthyratrons are also subject to a change in delay t1 over a relativelylong time interval. This drift may be measured vwhile measuring jitterby noting the change in position of the jitter pattern overy a long timeinterval.

The incremental jitter measuring device may also be used to measurepulse width jitter. This is accomplished by feeding both channels ofFig. 7 with the same pulses, by placing S2 in its upper position, and byan opposite setting of the polarity selecting switches S1, Fig. 6, inthe two channels. With this arrangement, the differentiating circuitsfollowing the switches produce positive pulses from the leading edges ofthe pulses in one channel and from the trailing edges of the pulses inthe other channel. The device then operates to measure the jitterbetween the positive pulses in the two channels.

I claim:

1. Apparatus for measuring the departure in time of a pulse with respectto a predetermined instant comprising means responsive to said pulse forgenerating a first wave having a fixed starting phase relative to saidpulse, means for generating a second wave of the same frequency as saidfirst wave and having a fixed starting phase relative to said instant,said Waves coexisting for a plurality of cycles, and means coupled toeach of said wave generating means for measuring the phase differencebetween said waves.

2. Apparatus for measuring the time jitter in a series of pulsesrelative to a predetermined corresponding series of instants, saidapparatus comprising means actuated by each of said pulses for producingfor a predetermined period a wave having a fixed starting phase relativeto said pulse that is the same for all pulses, means for producing for apredetermined period following each of said instants a wave of the samefrequency as said afore* mentioned wave and having a fixed startingphase relative to said instant that is uniform for all said instants,said predetermined periods being of such duration as to include aplurality of cycles of said waves and being at least in part coexistent,and means coupled to each of said wave producing means for measuring therange of fluctuation of the phase difference between said waves.

3. Apparatus for measuring the time jitter in a series of pulses ofnominally constant repetition frequency, said apparatus comprising meansfor producing a continuous reference wave having frequency equal to anexact large multiple of said pulse repetition frequency, means actuatedby each of said pulses for producing a wave having the same frequency assaid reference wave and a fixed starting phase relative to said pulsethat is the same for all pulses, said wave having a duration less thanthe interval between said pulses, and means coupled to each of said waveproducing means for measuring the range of fluctuation of the phasedifference between said waves.

4. Apparatus as claimed in claim 3 in which means connected to one ofsaid wave producing means are provided for adjusting the fixed phasedifference between said waves.

5. Apparatus for measuring the time jitter in a recurring nominallyconstant interval, said apparatus comprising means actuated at the startof said interval for generating a wave having a period short relative tosaid interval and a fixed starting phase relative to the start uring therange of fiuctuation of tween said waves.v

' are provided for adjusting afterA being delayed by said network,appear put circuit, means coupled 'to ofsaid interval thatis thesan'effor all intervals, vmeans actuated at the end of saidjinterval forgeneratingawave bf-"the sameffrequency as said afre'utentionedwave and`having 7a L'xedstarting phase relative to the yend, of said intervalthatis the same `fcirall intervals, said waves being coexistent yfor, aplurality yof cycles, and-means coupled to each of said wave generatingmeans for measthe phase difference be-v output circuits, comprisingmeans for applyingvr a vseries,

S eachV of: said 'andv having axed starting vphase relative to` :saidoutput ypulse that is the vsame for all., output pulses, saidfwavesbeingcoexistentkfor va.;.lixrality of cycles, means fortning` part`of'at least one of the 'couplings vbetween said wave kgenerating.meansand said input'and output circuits for 'y yadjusting the 'fixedphasedifierence :between said Waves,

and means coupled to each of said wave generating means for measuringthe range of ktiuctuatiou of the phase difference between said waves.

of electrical pulses to said input circuit, which pulses,vr

ink said outactuated by each of said input pulses for'generating anelectrical wave having a period short relative to said transmissiondelay and a xed'startng phaselrelative to said input pulse thatiisthesame for all input pu1ses,i i i y 929, abstract of application Ser.

means coupled to said output circuit and actuated by said vrinputcircuitr and References Cited in the tileof this patent i i' Hman:sTA'rEs liwi-titers MOTHER matematicask a ,v v

, iOlicial Gazette, July 19, 1949vo1. 624, No. 3, pag

`outputpulsesy for generating an electrical` wave of-thefsamefrequency'as said aforementioned wave i

